Inter-annual climate variability affects foraging behavior and nutritional state of thick-billed murres breeding in the southeastern Bering Sea

Warm oceanographic conditions of the continental shelf regions in the southeastern Bering Sea are associated with drastic increases in the abundance of juvenile walleye pollock Gadus chalcogrammus at shallow depths. We hypothesized that thick-billed murres Uria lomvia would benefit from these warm conditions by taking advantage of such an abundant prey resource available near their breeding colonies. We compiled a large dataset on the foraging behavior and nutritional state of murres breeding on St. George Island between 2003 and 2015. Murres foraged mostly on the continental shelf in warm years, but foraged in both on-shelf and off-shelf habitats in cold years. Shifts in foraging locations were associated with changes in diving depths. Nighttime foraging and daily diving effort increased during cold years, suggesting murres had to work more to obtain food under cold compared to warm conditions. Chick diets shifted from squid and benthic fishes in cold years to juvenile pollock in warm years. Foraging trip duration and reproductive success of birds were not affected by shifting oceanographic conditions, suggesting that murres behaviorally mediated the effects of inter-annual climate variability on their reproduction. However, this 'behavioral buffering' had associated costs, reflected in higher corticosterone concentrations in the blood of murres in cold compared to warm years, indicating that breeding birds incurred higher levels of nutritional stress under cold conditions. Our multiyear integrative study provides support that warmer conditions on the continental shelf might benefit piscivorous seabirds due to an increase in the availability of juvenile walleye pollock in the southeastern Bering Sea.